Beta-pleated-sheet crystals are among the most stable of protein secondary
structures, and are responsible for the remarkable physical properties of
many fibrous proteins, such as silk. Previous thinking was that
beta-pleated-sheet crystals in the dry solid state would not melt upon input
of heat energy alone. Indeed, at conventional heating rates ($\sim$1-50 $^{\circ}$C/min), silk exhibits its glass transition ($\sim$175 $^{\circ}$C), followed by cold crystallization, and then by immediate
thermal degradation beginning at about 225 $^{\circ}$C. Here we
demonstrate that beta-pleated-sheet crystals can melt directly from the
solid state to become random coils, helices, and turns. We use fast scanning
chip calorimetry at 2,000 K/s to avoid thermal degradation, and report the
first reversible thermal melting of protein beta-pleated-sheet crystals,
exemplified by silk fibroin. The similarity between thermal melting behavior
of lamellar crystals of synthetic polymers and beta-pleated-sheet crystals
is confirmed.